When you click the "Edit Source" button you are not editing the source, you are given a lovely UI to edit the details of the newsletter, so don't say "Edit source"
On a closer look, the patch and the task description seem to be about different issues - the patch removes the (disfunctional) View Source button, the task talks about Edit Source.
@Addshore do you know how to reproduce? I'd guess the user must have newsletter editing rights, and VisualEditor must be installed, and then its edit tab renaming functionality kicks in?
I would guess this has something to do with VE and the options to always show the edit source tab?
I don't remember now however and will not be in a position to try to reproduce for a few months.
Indeed the 'Edit source' tab comes from VE, we can easily change the text to 'Edit' using the hook SkinTemplateNavigation::Universal.
I noticed another bug with the tabs: View source is shown to a publisher without a 'newsletter-*' right instead of the Edit tab. The solution is to merge this patch which will sort who can edit a Newsletter page.